A conventional solenoid valve, whose type is the above one which is closed in a normal state, is explained with reference to FIG. 3. FIG. 3 is provided based on FIG. 4 of JP 2006-258154 A.
As shown in FIG. 3, a solenoid valve 50 disclosed in JP 2006-258154 A includes: a stationary core 60 having a gas introduction passage 61 inside the stationary core 60, and a movable core 70 having a gas flow passage 71 inside the movable core 70, the movable core 70 being movable toward the stationary core 60 when an electromagnetic force is applied to the movable core 70. A valve body 75 is mounted at an end of the movable core 70 away from the stationary core 60.
A columnar body 80 having a sleeve portion 81 configured to guide a movement of the movable core 70 is fixed to the stationary core 60. A gas discharge passage 82, which is closed when a valve body 75 sits thereon, is provided at an end of the columnar body 80 away from the stationary core 60. The gas discharge passage 82 is formed as a circular (cylindrical) hole.
Between the stationary core 60 and the movable core 70, there is provided a spring 65 configured to bias the movable core 70 in a direction in which the movable core 70 is moved away from the stationary core 60 in order to cause the valve body 75 to sit.
On the other hand, there is provided an electromagnetic coil 73 configured to apply the electromagnetic force to the movable core 70 in order to move the movable core 70 toward the stationary core 60 against a biasing force of the spring 65 such that the valve body 75 is released from the gas discharge passage 82.
The columnar body 80 is generally cylindrical, a region of the movable core 70 on a side of the valve body 75 has a smaller diameter than on a side of the movable core 60, so that a cylindrical gas accumulation space 82 is defined around the valve body 75.
In addition, the gas introduction passage 61 of the stationary core 60 and the gas flow passage 71 of the movable core 70 are configured to maintain a communication state thereof no matter how a relative positional relationship of the stationary core 60 and the movable core 70 is (that is, no matter how an extension or contraction state of the spring 65 is).
In addition, a region of the gas flow passage 71 on a side of the stationary core 60 is one inflow passage 71a having a circular section and extending in a direction in which the movable core 70 moves; and a region of the gas flow passage 71 on a side of the valve body 75 is two outflow passages 71b branched from the inflow passage 71a each of which has a circular section and extends in a direction perpendicular to the inflow passage 71a. 
As seen from FIG. 3, the diameter of the section of the inflow passage 71a and the diameter of the section of each outflow passage 71b are substantially the same. In addition, the diameter of the gas discharge passage 82 is also the same as these diameters. On the other hand, a radius difference between the outer diameter of the gas accumulation space 84 (the inner diameter of the columnar body 80) and the valve body 75 is about half the above diameters as seen along a line extended from an opening part of each outflow passage 71b in the direction in which the outflow passage 71b extends.
Next, an operation of the above conventional solenoid valve 50 is explained.
In a normal state, by means of the biasing force of the spring 65 provided between the stationary core 60 and the movable core 70, the movable core 70 is biased in the direction in which the movable core 70 is moved away from the stationary core 60, so that the valve body 75 sits on the gas discharge passage 82 and the gas discharge passage 82 is closed.
When a valve-opening instruction is inputted, the electromagnetic coil 73 is driven by a control unit (not shown). Thus, the electromagnetic coil 73 applies the electromagnetic force to the movable core 70, so that the movable core 70 is moved toward the stationary core 60 against the biasing force of the spring 65. As a result, the valve body 75 is released from the gas discharge passage 82, and the solenoid valve 50 is opened.
When a valve-closing instruction is inputted and the driving of the electromagnetic coil 73 is stopped, the electromagnetic force disappears and the movable core 70 is moved again away from the stationary core 60 by means of the biasing force of the spring 65. As a result, the valve body 75 sits on the gas discharge passage 82 again, and the gas discharge passage 82 is closed.
In view of a gas flow, in a normal state, a gas (normally, a pressurized gas) supplied into the gas introduction passage 61 fills the gas accumulation space 84 via the gas inflow passage 71a and the gas outflow passages 71b of the gas flow passage 71. However, the gas discharge passage 82 is closed by the valve body 75, so that the gas is not discharged via the gas discharge passage 82.
When a valve-opening instruction is inputted and the electromagnetic coil 73 is driven, the valve body 75 is released from the gas discharge passage 82, so that the gas which has filled the gas accumulation space 84 is discharged via the gas discharge passage 82.
Subsequently, when a valve-closing instruction is inputted and the driving of the electromagnetic coil 73 is stopped, the valve body 75 sits on the gas discharge passage 82 again and the gas discharge passage 82 is closed. Then, the gas flow is stopped at the gas accumulation space 64 (the gas discharge via the gas discharge passage 82 is stopped).
The above explained conventional solenoid valve 50 has such a simple structure that it is cheap to manufacture the same and it is relatively easy to install the same.
JP 2006-258154 A is a prior art document, as already explained above.